The Determination of the Composition of Complex Ions in Solution by a Spectrophotometric Method

Title: The Determination of the Composition of Complex Ions in Solution by a Spectrophotometric Method Objective: To determine the composition of the complex ion by method of continuous variation or Job’s method. Apparatus: 250 cm3 standard volumetric flask, Glass funnel, UV/Vis spectrometer Materials: Ammonium iron(III) sulphate, Salicylic acid, Hydrochloric acid

Results and Calculations: Calculation of the amount of Ammonium Iron (III) sulphate required: Concentration of solution prepared = 2×10-3 M Molecular weight of Ammonium Iron (III) sulphate = 482.25 g/mol Amount of Ammonium Iron (III) sulphate required for 250 mL = (No. of moles × Mol.wt × 250)/1000 = (2 × 10-3 × 482.25 × 250)/1000 = 0.2411 g Calculation of the amount of salicylic acid required: Concentration of solution prepared = 2×10-3 M Molecular weight of salicylic acid = 138.121 g/mol Amount of salicylic acid required for 250 mL = (No. of moles × Mol.wt × 250)/1000 = (2 × 10-3 × 138.21 × 250)/1000 = 0.0691 g

Weight of Ammonium Iron (III) sulphate used: 0.2428 g Weight of salicylic acid used: 0.0698 g Volume of Fe3+ /mL

Volume of salicylic

Mole Fraction of

Absorbance

acid /mL 25.00 22.50 20.00 17.50 15.00 12.50 10.00 7.50 5.00 2.50 0.00

0.00 2.50 5.00 7.50 10.00 12.50 15.00 17.50 20.00 22.50 25.00 V Fe +V V Fe mole fraction, x = ¿ ¿

Fe3+, x 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

( λ=500nm) 0.018 0.043 0.099 0.143 0.153 0.213 0.121 0.110 0.100 0.070 0.058

3+ ¿

salicylic acid 3+ ¿

0.1 =

V Fe 25 ¿

Fe3+¿ V¿

= 2.5 mL

3+ ¿

Graph of Absorbance ( λ=500nm) against Mole Fraction of Fe3+ 0.25 0.2 0.15 Absorabance

f(x) = - 0.52x^2 + 0.54x + 0.01 0.1 R² = 0.78

0.05 0 0 0.2 0.4 0.6 0.8 1 1.2 Mole Fraction of Fe3+

Mole fraction of salicylate, y =

y=

x 1−x 0.5 1−0.5

Absorbance ( λ=500nm) Polynomial (Absorbance ( λ=500nm))

=1 Ratio of Fe3+ : Salicylate= 0.5:1 = 1:2

Mn+ + yL

[MLy]n+ , where

Mn+ : Fe3+ L: salicylic acid [MLy]n+: complex formed Substitute y=1 into the equation, Mn+ + L

[ML]n+

Discussion: Continuous Variation Method(CVM) or Job’s method is based on the measurement of a series of solutions in which the molar concentrations of two reactants differ but their sum is constant. It is used to determine the stoichiometry of metal to ligand in complexes. For a reaction of the type, Mn+ + yL

[MLy]n+

the amount of complex ionic solution can be determined colorimetrically for various ratios of [Mn+] to [L]; the total concentration of metal ion and ligand is kept constant. Measurements of the absorbance at a suitable wavelength will show a maximum when the ratio of ligand to metal is equal to that in the complex. Job’s method of continuous variation will be used in conjunction with UV/Visible spectroscopy to establish the formula of the complex formed by Fe(III) and salicylic acid. Job’s method gives accurate results only under this circumstances : 1. The reaction is a quantitative and complete. 2. A single complex species is formed, and the species is the stable under the condition of the reaction. 3. The max for the complex species is known and is that a different wavelength from either the ligand or the metal ion. 4. The pH and ionic strength of the solution remain constant. Using this method, one makes a series of absorbance measurements in which the concentration of Fe(III) and salicylic acid are varied, while the total number of moles remains

constant. The requirement is by preparing solutions of Fe(III) and salicylic acid at identical concentration and then mixing them in various volume ratios, keeping the total volume constant. Under ideal reaction conditions, the maximum amount of the complex will form when the mole fraction of Fe(III) and salicylate are in the correct stoichiometric ratio. All other combinations result in the formation of less amounts of the complex. Since the product complex is colored, the absorbance or the solution mixture indicates the amount of the complex that has formed. As the concentration of one of the reactants, Fe3+, increased from zero, so does the amount of complex, so that the absorbance increased. The absorbance reached a maximum in the solution in which metal ion and ligand are in the same proportions as in the complex, since this solution will contain the highest concentration of complex. Further additions of metal ion give solutions that contain insufficient salicylic acid to complex with all the metal, so absorbance due to the complex will then decreased. The absorbance of each solution is measured at a suitable wavelength, 500 nm in this experiment. The absorbance is directly proportional to concentration. The higher the concentration of the complex, the higher the absorbance of it. The graph is linear. Furthermore, the plot of this method generated a line or lines that intersect when extrapolated. In the graph, the intersection is at the point where mole fraction of Iron (III) is at 0.5 and that of salicylate is 1. The ratio of Iron (III) to salicylate is 1:2. The maximum absorbance occurs at the molar ratio of the combining ratio of reactants, which is also taken to be the intersection of the extrapolated lines. If the complex absorbs less strongly than the reactants, a minimum would be observed. From the calculations, the ionic equation of this experiment, Mn+ + L [ML]n+ was shown that the maximum number of salicylate ions that can be bonded to the Fe3+ ions was 1. From the graph, we can see that the data was varied by decreased drastically after the equilibrium of [Fe3+]=[salicylate]. This may due to the solutions were oxidized by the air while waiting to be measured. This caused the reaction in the solutions fasten and by the time measured the solutions, the reactions were half completed so the absorbance measured were lower than the expected value. The formula deduced from the experimental Job curves may be in error if the total concentration of reactants is too low or not maintained constant. Therefore, in this experiment, the solutions we prepared may be not in consistent concentration when mixing two portions of different solutions into a same test tube. Working with two phase systems can reduced these errors. The Continuous Variation Method is not ideal if the reactants form more than one complex. It is not also very useful for complexes with many ligands. This method can be used under some favourable circumstances to determine the stability constant for a complex, since the deviation of the experimentally-determined curve from the extrapolated lines arises from

dissociation of the complex. However, a similar deviation can be caused by departures from Beer's law. There are a few precautions done while conducting the experiment. All the glasswares were thoroughly cleaned using distilled water in order to avoid contamination. It was also avoid the contamination of salicylic acid solution by Fe(III). Students were advised to wear lab coats, goggles and gloves when dealing with solutions. Besides, the solution was advised to be mixed when needed to measure. This is due to prevent the solutions oxidized while waiting for analysed.

Conclusion: The composition of the complex ion by method of continuous variation or Job’s method was determined. When the mole fraction of Fe3+ was 0.5, the mole fraction of salicylate ions was 1. The ionic equation was determined, that is, Mn+ + L [ML]n+. References: Experiment 2 Determination of the Composition of Iron-Phenanthroline Complex by Job’s Method, nd. Available from: < http://www.sci.nu.ac.th/chemistry/elearning/Experiment/Experiment%202.pdf> [Accessed on 1 April 2014] Spectrophotometric Determination of the Formula of a Coordination Complex and an Equilibrium Constant, 2001. Available from: < http://www.tarleton.edu/Faculty/alow/1084exp3.htm?> [Accessed on 2 April 2014]